Radiators for heating oil shale of the type shown in U.S. Pat. No. 4,140,179, have a coaxially fed dipole radiator. However, directivity in the vertical plane of the radiation pattern has been poor.
In addition, for large diameter dipole radiating elements, a practical coaxial line, whose characteristic impedance would match the radiating impedance of the dipole structure, requires a very small size inner conductor which limits power. Otherwise, the diameter of the outer conductor of the coaxial transmission line becomes very large, and the transmission line structure becomes unduly expensive. Thus, when the radiator supplied by the transmission line structure is at a substantial depth, RF heating of oil shale in situ can become uneconomic.